In recent years, ink-jet recording systems have been applied to various printing fields such as photography, various kinds of printing, graphic marking, and special printing such as color filters, since they make it possible to simply and conveniently produce images at low cost. Specifically, by employing ink-jet recording apparatuses which eject minute ink droplets and control them, ink-jet inks which result in a wider range of color reproduction, greater durability as well as higher ejection adaptability, and special paper sheets of which ink absorbability, color forming properties of colorants and surface glossiness are markedly improved, it has become possible to result in image quality comparable to conventional silver salt photography. Improvement in image quality of the current ink-jet recording systems have been achieved only by integration of ink-jet recording apparatuses, ink-jet inks and specialized kinds of paper.
However, in ink-jet systems which necessitate specialized paper, problems surface in which recording media are limited and the cost of the recording media increases. Consequently, a number of trials have been conducted in which recording is performed on media to be deposited, which are different from typical specialized kinds of paper. Listed as specific examples of these kinds of paper are a phase variation ink-jet system which employs a wax ink which is a solid at room temperature, a solvent based ink-jet system which employs an ink composed of rapid drying organic solvents as a major component, and an ultraviolet radiation curable ink-jet system in which crosslinking is performed employing actinic radiation such as ultraviolet radiation (hereinafter referred to as UV radiation).
In recent years, of these systems, the actinic radiation curable ink-jet system has received favorable attention due to its relatively low unpleasant odor and also to its rapid drying properties capable of performing recording on non-ink absorptive recording media, compared to solvent based ink-jet systems, and a number of UV radiation curable ink-jet inks are disclosed (refer, for example, to Patent Documents 1-6).
In view of fastness of formed images, frequently employed is an ink which is produced by dispersing pigments as a major component.
A common, but major problem in achieving stable dispersion characteristics during pigment dispersion is how pigments are adsorbed onto dispersing agents. When polar solvents are used, non-polar interaction is utilized, while when non-polar solvents are employed, polar intersection is utilized. For example, when water-based solvents are used, non-polar interaction is utilized, while when non-poplar solvents are employed, polar interaction is utilized.
Of these, during dispersion of solvent-based pigments, it is proposed to the use acid-base concept. Namely, when pigments are acidic, basic dispersing agents are employed, while when pigments are basic, acidic dispersing agents are used. From such a viewpoint, a number of proposals have been made in which characteristics of employed dispersing agents are specified (refer, for example, to Patent Documents 3 as well as 7-10). However, nothing has yet described with regard to oxidation of dispersing agents used for pigment dispersion, amine value, and characteristics of pigments which are dispersoid used in combination.
Generally, an actinic radiation curable (or a photocurable) ink-jet ink, though it is a solvent based system, is a system which makes it difficult to achieve the desired adsorption between pigments and dispersing agents due to the presence of polar groups derived from polymerizable groups. In ink-jet inks, poor dispersibility of pigments results in critical problems, compared to that of conventional pigments. An ink-jet recording system is a recording system in which liquid droplets containing pigment particles are ejected at a high rate from minute nozzles. Consequently, when dispersion is unstable, such unstable ejection results in critical problems. As a result, it is essential to select appropriate dispersing agents for employed pigments. However, no method has been discovered which overcomes the above problems.                (Patent Document 1) Japanese Patent Publication Open to Public Inspection (hereinafter referred to as JP-A) No. 2002-179967        (Patent Document 2) JP-A No. 2002-241654        (Patent Document 3) JP-A No. 2002-348478        (Patent Document 4) JP-A No. 2003-147233        (Patent Document 5) JP-A No. 2003-253155        (Patent Document 6) JP-A No. 2003-55563        (Patent Document 7) JP-A No. 5-262808        (Patent Document 8) JP-A No. 8-38875        (Patent Document 9) JP-A No. 2003-128966        